Reduction furnace

ABSTRACT

A vertical reduction furnace for the production of metallic iron by means of the direct reduction of iron ore (DRI), comprising an iron ore feed zone, an iron ore reduction zone, a metallic iron discharge zone ( 3 ), accumulation means ( 6 ) communicating at an inlet end with the metallic iron discharge zone ( 3 ) and at an outlet end with gas sealing means ( 7 ). The accumulation means may accumulate the metallic iron along with the off-gases from the reduction process.

FIELD OF THE INVENTION

The present invention relates to a reduction furnace, in particular areduction furnace for the production of metallic iron by means of thedirect reduction of iron ore, extracted from mines, at a temperaturethat is less than the melting point of the components.

PRIOR ART

The product obtained as a result of a direct reduction process is knownas DRI or “Direct Reduced Iron”. The direct reduction process convertsiron ore into highly metallized iron, in the form of lump ore, pelletsor a mixture of these components. The process produces reduced ironcontaining variable quantities of carbon in the form of Fe₃C. Thismaterial is the ideal foodstock for electric arc furnaces used in theproduction of high-quality steel. Direct reduction systems can thus bemade to integrate with systems upstream of electric steelmaking plants.

In existing systems, the hot DRI from the furnace or reactor, also knownas a shaft furnace, is usually briquetted or cooled in specific vessels,as it is extremely reactive in air at the temperature at which it leavesthe reduction furnace.

Transporting the DRI directly to the electric arc furnace is clearlyadvantageous but involves a number of requirements, namely:

-   -   the connection between the two furnaces must be rendered inert,        as the metal will re-oxidize when exposed to atmospheric air and        humidity with a significant loss of metallization;    -   the reduction and steelmaking plants must be separated by means        of a temperature buffer to keep the DRI at a suitable        temperature for being fed into the electric arc furnace;    -   the flow of hot material from the furnace must be metered;    -   heat loss must be kept to a minimum.

The separation of the reduction and steelmaking plants, by means of abuffer, is necessary because the electric arc furnace operates on abatch cycle basis, whereas direct reduction plants operate continuouslyand it is neither possible nor advantageous to stop the flow in thereduction furnace according to the amount of DRI that is used. Moreover,reduction plants run for long periods of time, usually without any shortscheduled stops during the year, generally being shut down once forapproximately 20 days.

Since electric arc furnaces must usually be shut down once a week and,for a variety of reasons, are subject to sudden stoppages, it has beennecessary to develop devices so that the production of DRI can continueeven in such conditions. This has been achieved by adding a coolingvessel in which the material is cooled to ambient temperature, or atleast to temperatures at which rapid re-oxidation is prevented, beforebeing discharged and stored ready for later use.

Note that the cooling system is not necessarily always associated withan arc furnace. Indeed, the cooling vessel can also be used to cool theDRI and render the plant independent of the end use.

For cases in which the material from the reduction furnace is fed intoan EAF at the same works, reduction systems providing for the directconnection between the reduction furnace and melting furnace fordischarging both hot and cold DRI simultaneously are known in the priorart. For instance, document WO 01/14598 describes a system with adynamic gas seal or seal leg between the reduction furnace and thebuffer. In this way the buffer is rendered inert thanks to the seal leg.

Once the point of delivery to the EAF has been defined in the meltingfurnace design, the presence of any vertical elements in the hightemperature connecting means/system will inevitably anddisadvantageously raise the height of installation of the reductionfurnace. For this reason systems comprising the lowest number ofvertical elements are preferable.

Disadvantageously, the presence of a seal leg means the reductionfurnace must be placed even higher up if the electric arc furnace (EAF)is gravity-fed. The reduction shaft, which is already high, is thus madeeven higher, increasing the costs of construction and installation atthe works.

Furthermore, this type of system results in loss of temperature of theDRI as the hot off-gases from the process are withheld in the reactorand do not reach the buffer. However, hot gas must be used to render thebuffer inert, to prevent any disadvantageous cooling of the DRI, andenergy is thus wasted in heating the inert gas.

It must also be possible to cut off the EAF hot charging device and coolthe DRI, in another specific system, while keeping the reduction systemoperational in all conditions.

The need is therefore felt to implement an innovative reduction furnacecapable of overcoming the drawbacks described above.

SUMMARY OF THE INVENTION

The main purpose of this invention is to produce a reduction furnacethat, by integrating the discharge, transfer and buffer functions in asloping pipe directly connected to the reduction furnace and the hotoff-gases from the process, is capable of reducing heat loss during thepassage from the reduction furnace to the electric arc furnace to aminimum, thus reducing electric furnace cycle times, i.e. reducingso-called tap to tap time, and thus reducing energy consumption. Aboveall this guarantees increased steel plant productivity because a shortertap to tap time means the same system can be used to produce morecastings.

The present invention therefore achieves the purposes described abovewith a reduction furnace, defining a longitudinal axis, for theproduction of metallic iron by means of the direct reduction of iron orethat, according to claim 1, comprises an iron ore feed zone, an iron orereduction zone, and a metallic iron discharge zone, characterised inthat it comprises accumulation means communicating at an inlet end withsaid metallic iron discharge area and at an output end with gas sealingmeans, said accumulation means being suitable for accumulating saidmetallic iron together with the off-gases from the reduction process.

The diameter and length of the pipe are functions of the volume of thebuffer required to separate the reduction and steelmaking plants. Theslope of the pipe, which must have an angle steeper than the actual DRIrepose angle, which is approximately 30°, is such to prevent bridging ofthe DRI in the pipe.

The furnace according to the present invention advantageously reducesthe amount of power required by the DRI cooling circuit in the reductionsystem and eliminates cold DRI handling and management costs.

One advantage of the invention is the fact that the hot DRI, produced inthe reduction furnace, is sent directly to the steelwork's electricfurnace while being maintained at a temperature of approximately 700° C.Furthermore, the system allows the DRI discharge to be controlled andsent simultaneously or alternately through a cooling vessel, so as toobtain cold DRI in a known manner. The portions of DRI to be sent hot tothe buffer or to the cooling vessel for cooling, can be metered asrequired.

Another advantage consists of the maximization of the discharge heightof the hot DRI, since by integrating the buffer in the reactor, thefinal discharge point can be placed at the maximum possible height,after the seal leg.

The transfer pipe involves the use of one less element compared to thereduction systems known in the prior art, with the elimination of thegas seal leg, and relative inert gas injection device, which connectsthe reactor to the hot inert vessel.

The claims attached hereto describe preferred embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of this invention will becomeclear from the following detailed description of a preferred, but notexclusive, embodiment of a reduction furnace, that is merelyillustrative and not limitative, with the help of the drawings that areattached hereto, in which:

FIG. 1 is a front view of a system comprising the furnace according tothe present invention;

FIG. 2 is a cross-section of a part of the furnace in FIG. 1;

FIG. 3 is a cross-section, on a plane orthogonal to that in FIG. 1, of adetail of the furnace according to the invention;

FIGS. 4 and 5 are cross-sections of the detail in FIG. 3 on the sameplane as FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

With reference to FIG. 1, a reduction shaft is illustrated. Saidreduction shaft comprises:

-   -   a reduction furnace or reactor 1;    -   a transfer pipe 6 integrated in the reactor 1;    -   a cooling vessel 5.

The furnace or reactor 1 produces hot DRI, starting from oxides in theform of pellets and/or lumps, which it discharges at a high temperaturethrough a zone, preferably a conically-shaped zone, into the transferpipe 6.

Said transfer pipe 6, through regulating means 3 that control the hotdischarge from the reactor 1, enables portions of DRI to be directedsimultaneously or alternately to an electric arc furnace and to thecooling vessel 5, and also acts as an accumulator, also referred to as a“buffer” in this description.

A collecting duct 4, inserted into the pipe 6 and having a diameterlarger than the base of the cone of the reactor 1, is directly connectedto the cooling vessel 5 and material passes through said duct at alltimes when the system is running. In the area 2 of the transfer pipe 6 apile of material builds up according to the natural repose anglethereof.

The cooling vessel 5 is in turn directly connected to a cold dischargedevice 10 via a dynamic gas seal leg 9. In this case there is thus asingle flow of material from the reactor 1, even from the charge hopper,to the base of the shaft or column. The activation of the dischargedevice 10 generates a flow of material along the entire height of thereduction shaft, while the repose angles in the area 2 remain unchanged.The area 2 thus becomes an open point at which it is possible and easyto pick up portions of hot DRI which can be diverted, discharged orforced out of the collecting duct 4 and thus towards the pipe or buffer6.

The hot discharge regulating means may comprise, for instance, a blade 3arranged transversely in relation to the flow of material from thereactor 1, controlled by a hydraulic cylinder. This blade 3 sweeps thepile of material in the area 2 with an oscillating movement, so thatportions of hot DRI can be discharged outside the duct 4 towards thetransfer pipe 6 and, at the same time, inside the duct 4 towards thecooling vessel 5.

The speed at which the blade 3 oscillates determines the control of theinput flow both to the cooling vessel 5 and to the transfer pipe 6. Whenin the home position the blade 3 is parked outside the pile of DRI anddoes not obstruct the flow towards the cooling vessel 5.

The blade 3 may be operated autonomously, or it may advantageously bejoined to a “flow promoter” 8 the function of which is to preventbridging in the end part of the discharge cone 1′ of the cone of thereactor 1. This basically consists of through pipes with wear-proofprotuberances that rotate by a few degrees about their own axisalternately in the two directions at very low frequency. This movementprevents bridging in the discharge cone 1′ and guarantees a goodmaterial flow. The flow promoters 8 are not metering systems. Thealternating movement of these elements can be likened in terms offrequency to that of the blade 3. The blade 3 can thereforeadvantageously be operated by appropriately connecting it to the actualaxis H of the flow promoter 8. The blade 3 regulates the amount ofmaterial that is discharged, while the flow promoter 8 promotes acorrect and full supply.

Whether the blade 3 is autonomous or fixed to the axis of the flowpromoter 8, in the home position it is always parked outside the pile ofDRI that builds up in the area 2 in the upper part of the transfer pipe6, and does not obstruct the flow of DRI towards the cooling vessel 5.

An advantageous alternative form of the regulating means, which allowsthe flow promoter 8 to remain active both when the blade 3 is in thehome position and when acting as a meter, is illustrated in FIGS. 3, 4and 5. In the home position the blade 3 continues to oscillate in anempty environment. For that purpose there are two actuators 11, 12, forinstance hydraulic actuators, that drive two levers 13, 14 that areintegral with the axis H of the flow promoter. With the blade 3 at workor in the home position only one of the two hydraulic actuators 11, 12is operational, while the hydraulic circuit of the other isshort-circuited so as not to offer mechanical resistance.

The reactor 1 can thus be discharged, simultaneously diverting a portionof the material to the transfer pipe 6 and a portion to the coolingvessel 5, depending on the way in which the devices 3 and 10 areactivated and controlled.

The hot DRI that collects inside the transfer pipe 6, set at a slope andwith an internal coating of refractory material, passes through theactual pipe together with the hot off-gases from the process. The pipe 6thus simultaneously performs the following functions:

-   -   it acts as a buffer between the continuous process reduction        furnace or reactor 1, and the batch process electric arc        furnace;    -   it maintains the correct temperature for feeding an electric arc        furnace.

The presence of the off-gases from the process in the transfer pipe 6advantageously eliminates the problem of DRI re-oxidation and alsopromotes heat retention.

The hot DRI that accumulates in the transfer pipe 6 is dischargedtowards the user, at fixed intervals of time, via a dynamic seal leg 7,which has the function of maintaining the off-gases from the processinside the pipe 6. The pipe/buffer is not thus inert. At the end of theseal leg 7, there is a second flow regulating device, for instance avibro-extractor, rotary valve, oscillating blade or the like.

At this point the hot DRI may be transported directly by gravity to theelectric arc furnace, if the systems are arranged very close together,or transferred to the electric arc furnace by means of an inert conveyorsystem.

During the design phase the diameter and length of the transfer pipe 6must be appropriately defined as a function of the volume of the bufferthat is needed in order to separate the reduction furnace and thesteelmaking plant.

The specific embodiments described in this document are not limitativeand this patent application covers all the alternative embodiments ofthe invention as set forth in the claims.

1. Reduction furnace (1), defining a longitudinal axis, for theproduction of metallic iron by means of a direct reduction process ofiron ore comprising an iron ore input zone, an iron ore reduction zone,and a metallic iron discharge zone, characterised in that it comprisesaccumulation means (6) communicating at an inlet end with said metalliciron discharge zone and at an outlet end with gas sealing means (7),said accumulation means (6) being suitable for accumulating saidmetallic iron along with the off-gases from the reduction process. 2.Reduction furnace according to claim 1, comprising a duct (4), insertedinto the metallic iron discharge zone, to feed said metallic iron into acooling vessel (5).
 3. Reduction furnace according to claim 2,comprising regulating means (3) for controlling a flow of metallic ironbeing discharged from said discharge zone of the furnace (1). 4.Reduction furnace according to claim 3, wherein said regulating means(3) direct said flow simultaneously or alternately to said accumulationmeans (6) and to said duct (4).
 5. Reduction furnace according to claim4, wherein said accumulation means (6) slope in relation to thelongitudinal axis of said reduction furnace (1).
 6. Reduction furnaceaccording to claim 5, wherein said accumulation means (6) are basicallycylindrical in shape and arranged at a slope of more than 30° inrelation to the longitudinal axis of the reduction furnace (1). 7.Reduction furnace according to claim 4, wherein said regulating meanscomprise a blade (3) arranged transversely in relation to the flow ofthe metallic iron, adapted to oscillate.
 8. Reduction furnace accordingto claim 7, wherein said regulating means comprise metallic iron flowpromoting means (8) associated with the blade (3) and that can beoperated in a reciprocally coordinated way to prevent bridging of themetallic iron in the lower end part of the reactor (1).